cmd/compile: clearer error message for unkeyed composite literal with unexported field

[pjebs]

I have a struct:

package df

type SortKey struct {
	Key interface{}
	SortDesc bool
	seriesIndex int
}

From an outside package:

TRY 1

z := df.SortKey{"n", false}

Compiler error: too few values in df.SortKey literal

TRY 2

z := df.SortKey{"n", false, 0}

Compiler error: implicit assignment of unexported field 'seriesIndex' in df.SortKey literal

You can't win. Perhaps it should just be disallowed for Go 2 - It messes up backwards compatibility anyway amongst minor versions of packages (eg. net/http.Request and Context situation).

[mvdan]

You cannot modify unexported fields from separate packages. This applies to composite literals too. The error seems clear enough to me.

[pjebs]

I'm not referring to the error description. I'm suggesting literal assignments should just be banned, or when an unexported field exists, initialised to zero value when attempting to do literal assignment (since you can't access the unexported field anyway).

Please reopen issue.

[mvdan]

I don't understand what you mean, but I'm going to reopen this for now.

[mvdan]

Also, please note that language changes should be proposals, as they need careful consideration. You should try to make your proposal as detailed and clear as possible.

[dpinela]

In your example, the solution would be to use a keyed literal.

Note that using an unkeyed composite literal of a struct type defined in another package is already explicitly recommended against, to preserve compatibility when such types add fields - the Go 1 compatibility promise specifically excludes such unkeyed literals, for example. There's even a proposal to disallow them altogether, and they're already a warning in vet.

[zigo101]

@pjebs
If you are the maintainer of the library package, then you can put a zero-size non-exported field in your struct type to avoid using non-keyed literals in user code.

[ianlancetaylor]

@mvdan Minor point: please don't add a NeedsDecision label to Go 2 issues. We're using that as a marker to be applied only after review. Thanks.